1. Field of the Invention
The present invention relates to a monitoring apparatus for measuring ballistocardiographic signal (BCG) using a multichannel sensor intended to analyze this multichannel data with a cepstrum method for heart inter beat interval (heart IBI) and heart rate variability (HRV) calculation. The invention further relates to a method for analyzing this multichannel data with a cepstrum method for heart inter beat interval (heart IBI) and heart rate variability (HRV) calculation.
2. Background of the Invention
Heart rate variability (HRV) is the rate at which heartbeat changes in time. A HRV analysis gives markers for activity of the autonomic nervous system (ANS) and is applied, for example, for sleep/wakefulness monitoring, stress analyses and also to obtain information about the activity and condition of the heart. The standard HRV analysis is composed of different frequency ranges, and as an example the high frequency component HRV-HF in the range of 0.15 . . . 0.4 Hz shows the cardiopulmonary coupling as a coherence between heart rate and respiration cycle. The rhythm of a healthy heart under resting conditions is actually surprisingly irregular. These moment-to-moment variations in the heart rate are easily overlooked when average heart rate is calculated. Defining the HRV requires accurate measurement of the heart inter beat interval, which is usually the electrocardiogram RR interval (ECG RRI) measured using a contact electrode.
The results of ECG RRI measurement are accurate and have a good resolution, but at least standard devices of ECG measurement require direct contact to the patient's skin or body via cables or wires. This is uncomfortable in use e.g. during sleep and it means constraining the patient's movement to prevent disconnecting the sensors, and also creates a danger of entanglement or strangulation due to the cables. Additionally, these devices are complex to operate and require a trained individual to function properly. It is neither practical nor desirable to monitor people with bulky and obtrusive equipment There is a need to monitor people in their normal environment, for example at home in bed, when driving a car or at work. Heart IBI based on BCG signal, on the other hand, can be measured almost anywhere and it is an unobtrusive and non-invasive method, in which single channel pressure sensitive sensors in a bed mattress give feasible results for average heart rate per minute without the need for a trained individual, or the danger of entanglement or strangulation, but the accuracy and resolution is not adequate for HRV analyses, and analyses are not possible, which is obviously a problem.
Non-contact pressure sensors are largely used to measure BCG signal for detecting heart beat unobtrusively. The algorithms behind the existing commercial non-invasive single channel BCG bed mattress sensors have not been published, but they are probably based on filtering and pulse envelope triggering of single channel BCG. Usually, average heart rate is calculated by counting up the heart beat cycles during a longer time period e.g. one minute. Pulse envelope triggering is not accurate method when the BCG pulse shape varies strongly, causing variation that is not dependent on the actual heart beat interval, and causing error in comparison with the ECG RR-interval. The methods applied with Fourier Transform use long time windows for Discrete Fourier Transform (DFT), e.g. 20 seconds, to average the variance in time domain, which also decreases the time resolution and prevents detection of individual heart IBI value.
Sleep analyses can obviously be also improved by monitoring the respiration parameters. Sleep laboratories use additional sensors like the respiratory inductive plethysmogram (RIP) or the airflow methods for this purpose. When using a pressure sensor for the BCG recording, the respiration signal can be extracted as a low frequency component of the BCG signal.
One monitoring method is disclosed in the publication: J. Alihanka, K. Vaahtoranta and I. Saarikivi, “A new method for long-term monitoring of the ballistocardiogram, heart rate, and respiration”, Am. J. Physiol. Regul. Integr. Comp. Physiol. 1981; vol. 240: 384-392, in which the principle of the static charge-sensitive-bed (SCSB) method is described. From the SCSB recording the BCG and respiratory movement can be recorded simultaneously by selective filtering of the original signal. The SCSB recording thus enables continuous long-term monitoring of the BCG, heart rate, respiratory rate, respiratory amplitude, and body movements e.g. during sleep. However, this method is not accurate enough for HRV analysis.
There exists a need for a multichannel sensor and a method for receiving more practicable non-contact BCG monitoring with better accuracy.